Photodiode

ABSTRACT

According to one embodiment, a photodiode includes a first semiconductor layer of a first conductivity type, a second semiconductor layer of a second conductivity type, a third semiconductor layer of the first conductivity type, and a film. The second semiconductor layer is provided in the first semiconductor layer. The third semiconductor layer is provided in the first semiconductor layer so as to surround the second semiconductor layer. Each of one ends of the second and third semiconductor layers is located at an upper surface of the first semiconductor layer. The first to third semiconductor layers include first to third impurity concentrations respectively. The second and third impurity concentrations are higher than the first impurity concentration. The film is provided above the third semiconductor layer, and blocks light to enter into a neighborhood of the third semiconductor layer.

CROSS REFERENCE TO RELATED APPLICATION

This application is based upon and claims the benefit of priority fromthe prior Japanese Patent Application 2013-186017, filed on Sep. 9,2013, the entire contents of which are incorporated herein by reference.

FIELD

Embodiments described herein are generally related to a photodiode.

BACKGROUND

In the back ground art, lateral pin photodiodes are known. The lateralpin photodiode has a p-type semiconductor layer, an i-type semiconductorlayer and an n-type semiconductor layer. The p-type semiconductor layerand the n-type semiconductor layer are disposed parallel to a surface ofa semiconductor substrate. The i-type semiconductor layer is interposedbetween the p-type semiconductor layer and the n-type semiconductorlayer.

The lateral pin photodiode detects light to enter into the i-typesemiconductor layer.

When the i-type semiconductor layer is a p-type semiconductor layer witha sufficiently low impurity concentration, the n-type semiconductorlayer and the i-type semiconductor layer form a pn-junction. A depletionlayer of the pn-junction extends to the p-type semiconductor layer siderather than the n-type semiconductor layer side. When light enters intothe i-type semiconductor layer, the light is absorbed inside the i-typesemiconductor layer to generate carrier.

The carrier which is generated inside the depletion layer flows as driftcurrent. The carrier which is generated outside the depletion layerflows as diffusion current. As a result, a sufficient response speed ofthe lateral pin photodiode is not obtained.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are views showing a photodiode according to anembodiment. FIG. 1A is the plan view of the photodiode. FIG. 1B is thecross-sectional view of the photodiode taken along a line A-A of FIG.1A.

FIG. 2 is a graph showing response characteristics of the photodiodeaccording to the embodiment in comparison with a photodiode of acomparative example.

FIGS. 3A and 3B are views showing the photodiode of the comparativeexample according to the embodiment. FIG. 3A is the plan view of thephotodiode. FIG. 3B is the cross-sectional view of the photodiode takenalong a line B-B of FIG. 3A.

FIGS. 4A, 4B, 5A, 5B and 6 are cross-sectional views showing steps ofmanufacturing the photodiode in order according to the embodiment.

FIG. 7 is a cross-sectional view showing another photodiode according tothe embodiment.

FIG. 8 is a cross-sectional view showing another photodiode according tothe embodiment.

DETAILED DESCRIPTION

According to one embodiment, a photodiode includes a first semiconductorlayer of a first conductivity type, a second semiconductor layer of asecond conductivity type, a third semiconductor layer of the firstconductivity type, and a film. The first semiconductor layer has a firstimpurity concentration. The second semiconductor layer is provided inthe first semiconductor layer, an one end of the second semiconductorlayer being located at an upper surface of the first semiconductorlayer, and has a second impurity concentration higher than the firstimpurity concentration. The third semiconductor layer is provided in thefirst semiconductor layer so as to surround the second semiconductorlayer, an one end of the second semiconductor layer being located at theupper surface of the first semiconductor layer, and has a third impurityconcentration higher than the first impurity concentration. The film isprovided above the third semiconductor layer, and blocks light to enterinto a neighborhood of the third semiconductor layer.

Embodiments will be described below with reference to the drawings. Inthe drawings, the same reference numerals show the same or similarportions. The same portions in the drawings are denoted by the samenumerals and a detailed explanation of the same portions isappropriately omitted, and different portions will be described.

Embodiment

A photodiode in accordance with an embodiment will be described withreference to FIGS. 1A and 1B. FIGS. 1A and 1B are views showing thephotodiode of the embodiment. FIG. 1A is the plan view of thephotodiode. FIG. 1B is the cross-sectional view of the photodiode takenalong a line A-A of FIG. 1A, viewed from the arrows.

The photodiode of the embodiment is a lateral pin-photodiode having ap-type semiconductor layer, an i-type semiconductor layer and a n-typesemiconductor layer. The p-type semiconductor layer and the n-typesemiconductor layer are disposed parallel to a surface of asemiconductor substrate. The i-type semiconductor layer is interposedbetween the p-type semiconductor layer and the n-type semiconductorlayer.

The lateral pin photodiode detects light to enter into the i-typesemiconductor layer. Hereinafter, the lateral pin photodiode is simplyreferred to as the photodiode.

As shown in FIGS. 1A and 1B, a photodiode 10 of the embodiment isprovided on a semiconductor substrate 11. The semiconductor substrate 11is a p-type (a first conductivity type) silicon substrate, for example.A p-type first semiconductor layer 12 is provided on the semiconductorsubstrate 11. The first semiconductor layer 12 has a first impurityconcentration of approximately 1E13 cm⁻³ and a thickness ofapproximately 10 μm, for example. Since the first impurity concentrationis low enough, the first semiconductor layer 12 is considered to be ani-type semiconductor layer.

An n-type (a second conductivity type) second semiconductor layer 13 isprovided in the first semiconductor layer 12, an one end of the secondsemiconductor layer 13 is located at an upper surface 12 a of the firstsemiconductor layer 12. The second semiconductor layer 13 has a shape ofcolumn, for example. The second semiconductor layer 13 has a secondimpurity concentration of approximately 1E18 to 1E19 cm⁻³, and athickness of approximately 3 to 4 μm, for example. The second impurityconcentration is higher than the first impurity concentration.

A p-type third semiconductor layer 14 is provided in the firstsemiconductor layer 12 so as to surround the second semiconductor layer13, an one end of the third semiconductor layer 14 is located at theupper surface 12 a of the first semiconductor layer 12. The thirdsemiconductor layer 14 has a third impurity concentration ofapproximately 1E18 to 1E19 cm⁻³, and a thickness of approximately 3 to 4μm, for example. The third impurity concentration is higher than thefirst impurity concentration.

The distance between the second semiconductor layers 13 is approximately10 to 20 μm. A width of the third semiconductor layer 14 isapproximately 1 to 2 μm.

The third semiconductor layer 14 has a hexagonal shape in which thesecond semiconductor layer 13 is center, for example. Two or more secondsemiconductor layers 13 and two or more third semiconductor layers 14are provided, and are arranged in a honeycomb shape.

The lateral pin photodiode 10 is formed of the third semiconductor layer14, the first semiconductor layer 12, and the second semiconductor layer13. The third semiconductor layer 14 is an anode of the photodiode 10,and the second semiconductor layer 13 is a cathode of the photodiode 10.

An insulating film 15 with translucency is provided on the firstsemiconductor layer 12, the second semiconductor layer 13, and the thirdsemiconductor layer 14. The insulating film 15 is a silicon dioxidefilm, for example.

A film 16 is provided above the third semiconductor layer 14.

The film 16 is provided on the third semiconductor layer 14 through theinsulating film 15. The film 16 is a metallic film, for example.

In plan view, the edge of the film 16 is outside the edge of the thirdsemiconductor layer 14 and inside an edge of a depletion layer. Thedepletion layer extends to the third semiconductor layer 14 side from apn junction of the first semiconductor layer 12 and the secondsemiconductor layer 13. A width of the depletion layer is dependent onthe first impurity concentration of the first semiconductor layer 12 andthe second impurity concentration of the second semiconductor layer 13,and is approximately 3 to 5 μm.

More specifically, a width of the film 16 is larger than the width ofthe third semiconductor layer 14.

Since the film 16 prevents a portion of light 19 a from entering intothe first semiconductor layer 12 when the edge of the film 16 is outsidethe edge of the depletion layer, a detection sensitivity of thephotodiode 10 becomes lower. Accordingly, it is not preferable that theedge of the film 16 is outside the edge of the depletion layer.

An electrode (a first electrode) 17 is provided on the insulating film15. The electrode 17 is electrically connected to the secondsemiconductor layer 13 through a via penetrating the insulating film 15.The electrode 17 is a cathode electrode of the photodiode 10. Anelectrode (a second electrode) not shown is electrically connected tothe third semiconductor layer 14. The second electrode is an anodeelectrode of the photodiode 10.

The light 19 which enters into the first semiconductor layer 12 isabsorbed inside the first semiconductor layer 12 to generate carrier(electron-hole pair). The photodiode 10 outputs the generated carrier asphoto current. The outputted photo current is taken into a signalprocessing circuit (not shown). The photo current sequentially reachesthe third semiconductor layer 14 from the second semiconductor layer 13by way of the electrode 17, the signal processing circuit, and theelectrode (the second electrode) which is not shown.

Much light 19 a among the light 19 enters into the first semiconductorlayer 12. Meanwhile, the film 16 prevents the light 19 b from enteringinto a neighborhood 18 of the third semiconductor layer 14.

The light 19 a which enters into the first semiconductor layer 12 isabsorbed inside the first semiconductor layer 12 to generate carrier.The generated carrier is accelerated by electric field in the depletionlayer of the pn junction to flow as drift current with high speed.

When a p-type semiconductor layer and an n-type semiconductor layer arejoined, conduction electron and hole couple in the pn junction to form adepletion layer in which majority carrier is lacking. In the depletionlayer, the n-type semiconductor layer side is charged in positive, andthe p-type semiconductor layer side is charged in negative. Accordingly,electric field to pull back electron and hole to the n-typesemiconductor layer and the p-type semiconductor layer respectively isgenerated, and voltage difference (diffusion potential) is generated atthe both ends of the depletion layer. When carrier is injected into thedepletion layer, the injected carrier serves as drift current whichflows in accordance with the electric field. The higher the impurityconcentration is, the larger the diffusion potential becomes.

When the pn junction is a step junction, the diffusion potential isexpressed as VD=(kT/q) ln (Na×Nd/ni²). K denotes Boltzmann constant, Tdenotes absolute temperature, q denotes electric charge, NA denotesacceptor density, ND denotes donor density, and ni denotes intrinsiccarrier density.

FIG. 2 is a graph showing response characteristics of the photodiode 10in comparison with a photodiode of a comparative example. FIGS. 3A and3B are views showing the photodiode of the comparative example. FIG. 3Ais the plan view of the photodiode. FIG. 3B is the cross-sectional viewof the photodiode taken along a line B-B of FIG. 3A, viewed from thearrows. Firstly, the photodiode of the comparative example will bedescribed.

As shown in FIGS. 3A and 3B, a photodiode 30 of a comparative examplehas the same fundamental structure as the photodiode 10 shown in FIGS.1A and 1B. The photodiode 30 is different from the photodiode 10 in thatthe photodiode 30 does not include the film 16.

Since the photodiode 30 does not include the film 16, the light 19 bamong the light 19 reaches a portion of the first semiconductor layer 12which is the neighborhood 18 of the third semiconductor layer 14. Thelight 19 b which reaches the first semiconductor layer 12 is absorbedinside the first semiconductor layer 12 to generate carrier.

Since the neighborhood 18 of the third semiconductor 14 is separatedfrom the edge of the depletion layer (transition region) of the pnjunction formed by the first semiconductor layer 11 and the secondsemiconductor layer 12, the neighborhood 18 of the third semiconductor14 is a region which is not sufficiently depleted. Accordingly, thecarrier which is generated in the neighborhood 18 of the thirdsemiconductor 14 is not affected by the electric field to flow asdiffusion current.

A distance between the second semiconductor layer 13 and the thirdsemiconductor layer 14 is approximately 5 to 10 μm, and is larger thanthe width of the depletion layer with approximately 3 to 5 μm. When thepn junction is a step junction, the width of the depletion layer isexpressed as W=√((2ε/q) (1/NA+1/ND) VD). ε denotes permittivity ofsemiconductor layer.

In FIG. 2, a broken line 21 indicates light signal which enters intophotodiodes 10, 30, solid line 22 indicates response characteristics ofthe photodiode 10, and a chain line 23 indicates responsecharacteristics of the photodiode 30.

As shown in FIG. 2, it is assumed that the light 19 with rectangularwave form is entered at time t1. A rise time of the photodiode 10 isapproximately same as a rise time of the photodiode 30.

It is assumed that the light 19 with rectangular wave form is cut off attime t2. A response of the photodiode 10 falls at time t3. A response ofthe photodiode 30 falls at time t4 longer than time t3. A fall time ofthe photodiode 10 is shorter than a fall time of the photodiode 30.

Since the photodiode 30 of the comparative example does not include thefilm 16, the carrier still remains in the neighborhood 18 of the thirdsemiconductor layer 14 after cutting off the light 19. Since theremaining carrier flows as diffusion current with slow speed, the risetime of the photodiode 30 becomes inevitably long.

On the other hand, since the photodiode 10 of the embodiment includesthe film 16, the carrier which remains in the neighborhood 18 of thethird semiconductor layer 14 after cutting off the light 19 does notexist. Accordingly, the diffusion current with slow speed does not flow.As a result, the film 16 enables the rise time of the photodiode 10 toshorten.

A method of manufacturing the photodiode 10 will be explained. FIGS. 4A,4B, 5A, 5B and 6 are cross-sectional views showing steps ofmanufacturing the photodiode 10 in order.

As shown in FIG. 4A, the first semiconductor layer 12 is epitaxiallygrown on the silicon substrate 11 by vapor phase growth method, forexample. dichlorosilane (SiH₂Cl₂) gas is used as a process gas, forexample. diborane (B₂H₆) gas is used as a dopant gas, for example.

As shown in FIG. 4B, a resist film 41 having an opening 41 a to expose aportion of the first semiconductor layer 12 in which the secondsemiconductor layer 13 is to be formed is formed on the firstsemiconductor layer 12 by photolithography method. The secondsemiconductor layer 13 is formed by implanting phosphor ion (P⁺) intothe portion of the first semiconductor layer 12 using the resist film 41as a mask.

As shown in FIG. 5A, a resist film 42 having a honeycomb shaped opening42 a to expose a portion of the first semiconductor layer 12 in whichthe third semiconductor layer 14 is to be formed is formed on the firstsemiconductor layer 12 by photolithography method. The thirdsemiconductor layer 14 is formed by implanting boron ion (B⁺) into theportion of the first semiconductor layer 12 using the resist film 42 asa mask.

As shown in FIG. 5B, a silicon oxide film as the insulating film 15 isformed on the first to third semiconductor layers 12, 13, 14 by chemicalvapor deposition (CVD) method, for example. An opening 44 to expose aportion of the second semiconductor layer 13 is formed in the insulatingfilm 15 by photolithography method.

As shown in FIG. 6, a metallic film 45 to fill the opening 44 and coverthe insulating film 15 is formed by sputtering method, for example. Themetallic film 45 is an aluminum film, for example. A resist film 46having an opening 46 a to expose a portion of the metallic film 45except a region in which the film 16 and the electrode 17 are to beformed is formed on the metallic film 45 by photolithography method.

The metallic film 45 is removed using the resist film 46 as a mask byreactive ion etching (RIE) method, for example. Accordingly, theelectrode 17 which is electrically connected to the second semiconductorlayer 13 and the film 16 which blocks the light 19 b entering into theneighborhood 18 of the third semiconductor layer 14 are simultaneouslyformed.

As described above, since the photodiode 10 of the embodiment has thefilm 16 to block the light 19 b which enters into the neighborhood 18 ofthe third semiconductor layer 14, the carrier is not generated into theneighborhood 18 of the third semiconductor layer 14. Accordingly, sincethe carrier which remains into the neighborhood 18 of the thirdsemiconductor layer 14 when the light 19 is cut off does not exist, thefall time of the photodiode 10 can be shortened. As a result, aphotodiode with a fast response speed is obtained.

The description has been made here as to the case where the thirdsemiconductor layer 14 has the honeycomb shape. However, another shapesuch as a ring shape and a grid shape may be available as long as thethird semiconductor layer 14 surrounds the second semiconductor layer13. Since a distance between the second semiconductor layer 13 and thethird semiconductor layer 14 becomes constant when the thirdsemiconductor layer 14 has the ring shape, a distance between the thirdsemiconductor layer 14 and the edge of the depletion layer also becomesconstant. An advantage that a margin of the film 16 is increased isobtained. The margin means an acceptable range of the position of theedge of the film 16, for example.

The description has been made as to the case where a reverse biasvoltage is not applied to the photodiode 10. However, the reverse biasvoltage may be applied to the photodiode 10. Since the depletion layerfurther extends to the third semiconductor layer 14 side by applyingreverse bias voltage to the photodiode 10, an advantage that the marginof the film 16 is further increased is obtained.

The description has been made as to the case where the firstconductivity type is p-type and the second conductivity type is n-type.However, the same advantage may be obtained when the first conductivitytype is the n-type and the second conductivity type is the p-type. FIG.7 is a cross-sectional view showing a photodiode 50 in which the firstconductivity type is the n-type and the second conductivity type is thep-type.

The photodiode 50 is the same as the photodiode 10 shown in FIGS. 1A and1B except the conductivity type. The explanation of the photodiode 50 isomitted.

A shield layer to shield electromagnetic noise may be provided on thephotodiode. FIG. 8 is a cross-sectional view showing a photodiode havinga shield layer.

As shown in FIG. 8, in a photodiode 60, a p-type fourth semiconductorlayer (a shield layer) 61 is provided in the first semiconductor layer12, an one end of the fourth semiconductor layer 61 is located at theupper surface 12 a of the first semiconductor layer 12, the thirdsemiconductor layer 14 is provided in the first semiconductor layer 12,an one end of the third semiconductor layer 14 is located at the otherend of the fourth semiconductor layer 61.

A width of the fourth semiconductor layer 61 is larger than the width ofthe third semiconductor layer 14. The fourth semiconductor layer 61 hasa fourth impurity concentration of approximately 1E18 cm⁻³ and athickness of approximately 0.2 μm. The fourth impurity concentration ishigher than the first impurity concentration. The width of the fourthsemiconductor layer 61 is not especially limited unless the fourthsemiconductor layer 61 is in contact with the second semiconductor layer13.

The fourth semiconductor layer 61 and the substrate 11 are connected toground (a common voltage line). Since the first semiconductor layer 12is interposed between the fourth semiconductor layer 61 and thesubstrate 11 which are connected to the ground, the electromagneticnoise is prevented from entering into the first semiconductor layer 12.The fourth semiconductor layer 61 functions as a shield layer to shieldthe electromagnetic noise.

Since the fourth semiconductor layer 61 has the width within a range inwhich absorption of the light 19 is disregarded, the fourthsemiconductor layer 61 does not affect a detection sensitivity of thephotodiode 60.

While certain embodiments have been described, these embodiments havebeen presented by way of example only, and are not intended to limit thescope of the inventions. Indeed, the novel embodiments described hereinmay be embodied in a variety of other forms; furthermore, variousomissions, substitutions and changes in the form of the embodimentsdescribed herein may be made without departing from the spirit of theinventions. The accompanying claims and their equivalents are intendedto cover such forms or modifications as would fall within the scope andspirit of the inventions.

What is claimed is:
 1. A photodiode, comprising: a first semiconductorlayer of a first conductivity type having a first impurityconcentration; a second semiconductor layer of a second conductivitytype provided in the first semiconductor layer, one end of the secondsemiconductor layer being located at an upper surface of the firstsemiconductor layer, and having a second impurity concentration higherthan the first impurity concentration; a third semiconductor layer ofthe first conductivity type provided in the first semiconductor layer soas to surround the second semiconductor layer, one end of the thirdsemiconductor layer being located at the upper surface of the firstsemiconductor layer, and having a third impurity concentration higherthan the first impurity concentration; and a film provided above thethird semiconductor layer, and blocking light to enter into aneighborhood of the third semiconductor layer.
 2. The photodiodeaccording to claim 1, wherein a width of the film is larger than a widthof the third semiconductor layer, and an edge of the film is in thethird semiconductor layer side from an edge of a depletion layer, thedepletion layer extending to the third semiconductor layer side from apn junction of the first and second semiconductor layers.
 3. Thephotodiode according to claim 1, wherein the third semiconductor layerwith a polygon shape or a circle shape surrounds the secondsemiconductor layer in a shape of polygon or circle.
 4. The photodiodeaccording to claim 1, further comprising a fourth semiconductor layer ofthe first conductivity type provided in the first semiconductor layer,one end of the fourth semiconductor layer being located at the uppersurface of the first semiconductor layer, and having a width larger thana width of the third semiconductor layer and a fourth impurityconcentration higher than the first impurity concentration, wherein theend of the third semiconductor layer is located at a lower surface ofthe fourth semiconductor layer.
 5. The photodiode according to claim 4,wherein the fourth semiconductor layer is electrically connected to acommon voltage line.
 6. The photodiode according to claim 1, wherein thefirst semiconductor layer is provided on a semiconductor substrate ofthe first conductivity type.
 7. The photodiode according to claim 6,wherein the semiconductor substrate is electrically connected to acommon voltage line.
 8. The photodiode according to claim 1, furthercomprising: a first electrode electrically connected to the secondsemiconductor layer; and a second electrode electrically connected tothe third semiconductor layer.
 9. The photodiode according to claim 1,wherein the third semiconductor layer is substantially equal to thefirst semiconductor layer in thickness.
 10. The photodiode according toclaim 1, further comprising an insulating film with translucencyprovided on at least the first and third semiconductor layers, whereinthe film is provided on the insulating film.
 11. The photodiodeaccording to claim 1, wherein the film surrounds the secondsemiconductor layer in plan view.
 12. A photodiode, comprising: a firstsemiconductor layer of a first conductivity type having a first impurityconcentration; a second semiconductor layer of a second conductivitytype provided in the first semiconductor layer, one end of the secondsemiconductor layer being located at an upper surface of the firstsemiconductor layer, and having a second impurity concentration higherthan the first impurity concentration; a third semiconductor layer ofthe first conductivity type provided in the first semiconductor layer soas to surround the second semiconductor layer, one end of the thirdsemiconductor layer being located below the upper surface of the firstsemiconductor layer and having a third impurity concentration higherthan the first impurity concentration; a film provided above the thirdsemiconductor layer, and blocking light to enter into a neighborhood ofthe third semiconductor layer; and a fourth semiconductor layer of thefirst conductivity type provided in the first semiconductor layer, oneend of the fourth semiconductor layer being located at the upper surfaceof the first semiconductor layer, the other end of the fourthsemiconductor layer reaching the one end of the third semiconductorlayer, and having a width larger than a width of the third semiconductorlayer and a fourth impurity concentration higher than the first impurityconcentration.
 13. The photodiode according to claim 12, wherein a widthof the film is larger than the width of the third semiconductor layer,and an edge of the film is in the third semiconductor layer side from anedge of a depletion layer, the depletion layer extending to the thirdsemiconductor layer side from a pn junction of the first and secondsemiconductor layers.
 14. The photodiode according to claim 12, whereinthe third semiconductor layer with a polygon shape or a circle shapesurrounds the second semiconductor layer.
 15. The photodiode accordingto claim 12, wherein the fourth semiconductor layer is electricallyconnected to a common voltage line.
 16. The photodiode according toclaim 12, wherein the first semiconductor layer is provided on asemiconductor substrate of the first conductivity type.
 17. Thephotodiode according to claim 16, wherein the semiconductor substrate iselectrically connected to the common voltage line.
 18. The photodiodeaccording to claim 12, further comprising: a first electrodeelectrically connected to the second semiconductor layer; and a secondelectrode electrically connected to the third semiconductor layer. 19.The photodiode according to claim 12, wherein the third semiconductorlayer is substantially equal to the first semiconductor layer inthickness.
 20. The photodiode according to claim 12, further comprisingan insulating film with translucency provided on at least the first andfourth semiconductor layers, wherein the film is provided on theinsulating film.
 21. A photodiode, comprising: a first semiconductorlayer of a first conductivity type having a first impurityconcentration; a second semiconductor layer of a second conductivitytype provided in the first semiconductor layer, one end of the secondsemiconductor layer being located at an upper surface of the firstsemiconductor layer, and having a second impurity concentration higherthan the first impurity concentration; a third semiconductor layer ofthe first conductivity type provided in the first semiconductor layer soas to surround the second semiconductor layer, one end of the thirdsemiconductor layer being located at the upper surface of the firstsemiconductor layer, and having a third impurity concentration higherthan the first impurity concentration; and a film provided above thethird semiconductor layer so as to surround the second semiconductorlayer in plain view, and a width of the film being larger than a widthof the third semiconductor layer.